Various N900 improvements, change default i3 terminal to "st"

So I got myself a N900, and I'm going to turn it into a nice music player with postmarketOS. That gives me a good opportunity for dogfooding. Three commits incoming, I put them into one MR so they can be tested together.

postmarketos-ui-i3wm: use st, not xfce4-terminal

st is the "simple terminal" from suckless. It starts up a bit faster (on the N900 you can actually notice it) and has less dependencies.

There's no menubar or scrollbar by default, which is also nice for small screens.

postmarketos-artwork-wallpapers: update to latest

Adds a new 800x480 version of the grass wallpaper, suitable for the Nokia N900.

device-nokia-n900-i3wm: various tweaks

• set the postmarketOS grass wallpaper
• add a new "mpd" mode to the i3wm config (NOTE: mpd is not installed by default)
• add conky to display "postmarketOS" and the kernel version on the desktop (changes to playback information when a song is playing with mpd; easy to customize by editing the conky config)
• hide the mouse cursor (by starting unclutter-xfixes)
• display 'PROTIP: ^i (ctrl+i) does autocompletion' when starting a terminal

important note

When testing this MR, you need to manually trigger a build for postmarketos-artwork-wallpapers. pmbootstrap does not check if it need to be updated, as it is only a are dependency of a subpackage.

(please do not squash!)